Electrically noiseless progressive contact means



\ Nov. 13, 1951 R. P. MORK 2,575,230

ELECTRICALLY NOISELEISS PROGRESSIVE CONTACT MEANS Filed July :5, 1948 2 SHEETS-SHEET 1 Q o HG,-

INVENTOR. fray/valve P MORK,

ATTORNEYS.

Nov. 13, 1951 'pj MQRK 2,575,230

ELECTRICALLY NOISELESS PROGRESSIVE CONTACT MEANS Filed July 3, 1948 2 SHEETS-SHEET 2 13 19 174/ 17 Z9 o o 12' 1 33 2.9 32 o o 5 i fi pil 32 -QMMQM ATTORN EYS.

Patented Nov. 13, 1951 ELECTRICALLY NOISELES'S PROGRESSIVE CONTACT MEANS Raymond P. Mork, Cincinnati, Ohio, assignor to The Baldwin Company, Cincinnati, Ohio, a corporation of Ohio.

Application July 3, 1948, Serial No. 36,886

13 Claims. 1

My invention relates to progressive electric contact means such as may be employed with an electric resistance element to provide either a continuously variable resistor or a potentiometer.

The primary object of my invention is to provide a durable, economical and electrically nise less progressive contact means.

Another object of my invention is to provide ar electrically noiseless, continuously variable resistance device or potentiometer which is particularly adaptable to circuits carrying small electric currents such as occur in electronic devices, radio receivers, electric musical instruments, electric measuring equipment and the like.

A further object of my invention is to provide variable resistance means in which minor irregularities in the surface of a resistance element and/or the presence of foreign particles on a resistance element do not result in abrupt resistance changes, and hence noise in circuits which feed amplifiers and loud speakers.

A specific object of my invention i the provision of simple, inexpensive, dependable and durable apparatus suitable for use as a pedal actuated volume control for electrical musical instruments.

These and other objects of my invention which will be set forth hereinafter, or will be apparent to one skilled in the art from the following teachings, I attain by that construction and arrangement of parts and in that mode of operation of which I shall now describe an exemplary embodiment. Reference is made to the accompanying drawings wherein:

Figure l is a plan view of a resistance element disposed upon the base of a variable resistance device in accordance with my invention.

Figure 2 i a plan view of a contact strip employed in my invention.

Figure 3 shows the underside of the contact strip of Figure 2.

Figure 4 illustrates a plan view of a potentiometer, as an embodiment of my invention, employing the elements of Figure 1 and Figure 2.

Figure 5 is an elevational view of the structure of Figure 4.

Figure 6 is an enlarged sectional view 6-6 of Figure 5.

Figure 7 is a portion of a variable resistor according to my invention.

Figure 8 is a plan view of a volume control apparatus with part of the cover broken away.

Figure 9 is a side elevation thereof.

Figure 10 is a plan view from the reverse side.

Figure 11 is a partial longitudinal sectional view taken along the section line H-Il of Figure 10.

Figure 12 is a transverse sectional view taken along the section line |2I2 of Figure 11.

Figure 13 is a perspective view of a bearing block.

Figure 14 is a view, partly in elevation and partly in section showing the volume control device in use as a pedal-actuated swell control in an electrical musical instrument.

Briefly, according to my invention, I employ elastically deformable, electrically conductive material interposed between a resistance element and a contactor, which material is caused to contact the electrical resistance element progressively and without tangential relative motion at the area of contact. In so doing, I not only eliminate friction between the contact surfaces, thus minimizing wear, but also I provide a contact surface which by virtue of its own deformability makes intimate contact with the resistance element in spite of minor irregularities in the resistance element or the contactor and/or in spite of the presence of foreign particles thereon, both of which conditions, when present in known practical devices of this general type, may cause abrupt changes in the electrical resistance offered by such devices when a resistance variation is effected. Such abrupt changes are often referred to as electrical noise, because of the crackling sound produced when such devices are employed to vary currents in electroacoustic systems.

I shall now describe a progressive electrical contact means in accordance with my invention and capable of general use, after which I shall show it in use as a volume control for an electrical musical instrument.

In Fig-ure 1 an area I of an insulative base 2 of a phenolic or other suitable material may be coated with a layer of a carbonaceous or other material having a desired resistivity. Areas 3 and 3a, preferably of a conductive lacquer or a thin metallic layer partially underlying the ends latter strip is indicated at 6 in Figure 3, which.

shows the underside of theassembly 4 of Fi 4 not of importance except as explained below, since the current is required to flow only through the thinnest section of the layer 6.

It will be obvious to those skilled in the art that the illustrated embodiment of my invention may be employed either as a variable resistor, in

-';which"case1- connections can": be' either: between ure 2. Other modes of assembly are practicable,

such as vulcanizing a thin layer of the rubber substance to a sheet of phosphor bronze andcut- V i employing a terminal screw I5, as shown in ting the sheet into strips. "It is within the scope of my invention to employ a strip of cloth or other mesh fabric impregnated 'andcoatedwith the conductive elastomer.

The elements of Figures 1 and 2 may beessembled into a potentiometer such as that shown i1iEFigures 4' and 5, .yvherein thejelementsbear the same indicia as in the preceding figures." The :contacta member '4' :mayr-bef fastened by means of a screw'iTto Ethe base 2, *being spaced slightly between the contact member 4 and the resistive izcoating' 1 maybemaintainedby a tension spring 9;one end thereof 'engaging'a slot II] in the mem- :and acts as a guide to prevent'tr'ansverse motion ithereof. Screws I3 and I3a, and associated nuts provide means, in' contact with the areas 3 and therefromi'by a washer iA- 'spaced relationship was-desired, the area ofcontact between the layer --3a','?for. making electrical connections tothe remay; also employ'arolling actuatorin the'place &5

: oftheslidingone described above. 'Such'a roller Yisillustrated at I6 inFigure'I. a

c: Figure 6 is'an enlarged sectional view'fi -fi of Figure 5' showin approximate-relative 'sizes of theyarious elements described'above. 1

- aThe illustrated structureis one irr'which there is a minimum of sliding-motion between thestrip T5 and the resistive coating'I makingfor long-life of these elements. The interposition of the resilient, deformable electrically conductive substance between *the'strip-S'and resistive coating I has the effect of minimizingwear still further, 'but -more importantly it has the effect of conforming by slight deformatiom to the "surface irregularities of theresistive coating, and of contacting that surface inspite of slight amounts of Ioreign materials thereon, so that good, positive electrical contact is-made-atall'times, and so that, as the position.of ;0ontac t is varied bymovement of; the actuator; I 4, abrupt-sporadic changes .in,;resistancewill not occur. Thus; electrical ggoise is eliminated i The efiectoccurs irrespective of the area of; contact1as determined by the sh pe Of theactuator I4; Since the irregularities in the-surface'of the resistive coating I are minute, n0, sreatdeformability isrequired in the layer 6. Hence, this layer can be very thin, say .cietheeo deeoffi r olsi-thousandths of-an; inch.

base 2. structure of Figure '7, may be similar to that of -sufiicient force between-the roller instrument.

. the screws I. and I3 or .I and I3a, or as a potentiometer, in which case connections can be made to screws I, I3 and I 3a in the manner usually ollowed in this art. However, if I wish to providega structurei'or use only as a variable resistor, I prefer to eliminate the screw I3 near the left hand end of the structure of Figures 4 and Figure:7 ;to connect the contact means 4 to the 1ef-t=hand-. .end .of the resistive element I on the The right-hand end, not shown, of the Figures 4 and 5. Connections for obtaining. a variable resistance maybe made to the terminal :screws I 5 and I3a; "Also in Figure '7, I have shown ga rigid roller I6 which I may employ, if I so de- .-sir e;inthe place of the actuator I4; By increasingthe diameter; of the roller I6, I may vary,

6 and the resistivez element I However, if I wish to obtain a relatively large area of contact with arelativ'ely small diameter roller, I may employ a roller-of a soft rubber-like material, applying a I6 and the resistive element I to flatten the periphery of the roller at its area of contact with the strip 5, thus causing a larger area of thelayer 6 to contact :the resistive element I.

t In Figures 8 to 11, I have shown an embodiment of my invention suitable'for useas a pedal actuatedvolume control in an electrical musical In this embodiment, .I provide a "channel shaped metal body II having flanges Ila at the ends 'of'thelegs of the channel. The channel is provided with a removable cover I9 "which may be held 'in' place by screws I8 or :the like engaging the flanges IIa.

:' Thebase of the channel is provided'with openings such as 20, 2 I, and 22 in Figure 10 through which the means for making electrical contact with the potentiometer are accessible. Thus at 23 and 23a, I have in'dicated' contact lugs to which soldered connections may be made which lugs may be used in connection with or in place of terminal screws I3 and 'I3a'in Figure 4. The 'baseof the channel may also be provided with other openings such as 24 to permitthe base plate 2 to lie snugly against "the base 'of the 'chann'elin spite of projections on the bottom side of"the base plate such as the rigid ends 'ofthe'posts II and I2. 1 prefer, however, to line my channel with a thin'layer of flexible insulativesheet' material 25'; which also serves in cooperation with other parts to provide a dusttight-"construction. rr'The'cover plate lamay 'al'so be lined 'with'a layer preferably of felt'26 or other suitable porous-material which maybeiimpregnated with a suitable lubricant. ".3: The; ends of the channel": are closed by means of blocks 21, preferably though not necessarily of 'woodnwith the grain in the direction oithe arrow in Figure 13. These'blocks may be impregnated with" a suitable/lubricant such as graphite: dispersed. inah oily vehicle; The-top surface of. each'blockisprovidedwith a groove held in place at the ends of the channel by screws or the like 30, with a suitable clearance for the passage of the bar 29. At the bottom. to permit expansion of the wood blocks and to contribute to dust-tightness, I may provide a layer of felt 3| or other suitable resilient substance.

The sliding bar 29, which may if desired be provided with rivets or other abutment members 32 to limit its motion within the channel, lies in the grooves 28 of the blocks 21 and beneath the cover element i9. An extendin end of the bar is perforated as at 33 or otherwise provided with means whereby it may be connected with actuating mechanism. Within the channel, I provide a spring element 34, one end of which is attached to the bar 29 and the other end of which bears an actuating element which will bear against the phosphor bronze strip 5. As shown in Figure 11, an exemplary construction comprises a block 35 of insulative material which carries a rounded carbon element Mo to slide along the strip. Other constructions may be employed. The spring 34 presses the actuator element [4a into contact with the strip 5 with suiiicient pressure to deflect the layer 6 against the resistive coating I.

For mounting the structure of Figures 8 to 12 in an electrical musical instrument, I prefer to provide a spring leaf member 39 which at one end is riveted or otherwise fastened against a side leg of the channel shaped body l1. At the other end, it is provided with one or more perforations 31, whereby it may be screwedor bolted against a suitable support such as the base member 38 of a musical instrument, as shown in Figure 14. Here the base member is provided with a mounting block 39 to which a pedal 40 is pivoted as at 4|. The bar 29 of the potentiometer device is pivoted to a bracket 42 on the underside of the pedal 40. As is known in the art, the pivoting elementsfor the pedal are preferably so constructed as to provide stops limiting the motion of the pedal. The spring 36 not only mounts my potentiometer in position as shown, but permits the potentiometer structure to swing in order to accommodate the rocking motion of the pedal. Connections are made to the potentiometer by means of a flexible cable or leads indicated generally at 43.

The operation is steady, devoid of lost motion, smooth in action, and noiseless. It is inherently lubricated and essentially dust-proof.

Regarding the electrical properties of the various elements described herein as an exemplary volume control, a practical device I have constructed employs a coating I having a resistance of approximately 500 ohms between the terminal screws [3 and l3a. The electrically conductive strip 6, is of a commercially available conductive rubber having a thickness of approximately .015 inch and an electrical resistivity of approximately 2 ohm-centimeters. The strip 5 is of .005 inch phosphor bronze, with a rigid graphite actuator I4. I may prefer in the case of a relatively low resistance control, such as the one just described, to cover the elastically deformable strip 6, and for the resistive coating I, with a thin coating of colloidal carbon applied in suspension in a liquid vehicle which subsequently evaporates. Such a coating has been found beneficial due to the fact that after prolonged use, the strip 6 increases in surface resistivity. This thin coating helps to 6 stabilize the resistivity of the layer 6 at a desired low value.

In the case, however, of a structure employing a coating I having a relatively high resistance between terminalssay 10,000 ohms, such a surface coating is not so necessary for the deformable strip 6. If a material having inherently stable resistivity in spite of prolonged use can be found, a low resistivity surface coating can probably be dispensed with entirely.

In its broader aspects, my invention is not limited to the embodiment herein disclosed. For instance it is within the scope of my invention to employ a circularly disposed resistance element, a similarly shaped contact element, and a rotating actuating means for progressively shifting the area 7 contact between a conductive elastomer and a resistance element.

Modifications may be made in my invention Without departing from the spirit of it. Having thus described my invention in an exemplary embodiment, what I claim as new and desire to secure by Letters Patent is:

1. A variable resistance device comprising in combination a resistance element, a contact unit comprising a resilient, flexible metallic strip having an elastically deformable, electrically conductive layer disposed upon a surface thereof adjacent said element, and means operative upon said member for forcing successive portions of said layer into contact with said element.

2. The combination claimed in claim 1, in which said resistance element comprises a layer of resistive material disposed upon an insulative base member having terminals mounted thereon for making connections to said resistance element and to said contact unit.

3. A variable resistance device comprising in combination an elongated insulative base having a resistive coating disposed longitudinally thereupon, terminal means connected to the ends of said coating, a contact unit having an end affixed to said base, terminal means for said end, said unit comprising a flexible, resilient metallic strip having an elastically deformable, electrically conductive layer disposed thereon, means in connection with the other end of said contact unit normally maintaining at least a portion of said layer in a spaced relationship with respect to said resistive coating, and means operative upon said strip for forcing successive portions of said layer into contact with said coating.

4. A potentiometer device comprising in combination an elongated insulative base having a resistance coating disposed longitudinally thereon, terminal means electrically connected to the ends of said coating, a contact unit having an end spacedly aflixed to said base, terminal means for said end, said unit comprising a flexible, resilient, metallic strip having an elastically deformable, electrically conductive layer disposed thereupon, means in connection with the other end of said contact unit tending to maintain said layer in a spaced relationship with respect to said resistive coating, and means operative upon said strip for forcing successive portions of said layer into contact with said coating.

5. In a variable resistance device, a base having an elongated rigid resistive element thereon, a resilient, flexible metallic member normally lying above said element, but depressible into contact therewith at various points throughout its length, and, interposed between said member and element, a resilient, flexible, deformable, electrically conductive substance acting, when said member and element are pressed togetherfto increase the area of effective electricalicontact therebetween.

channel-shaped body, a cover therefor, grooved blocks closing the ends of the channel-shaped body, a bar slidable in the grooves of said blocks and beneath said cover, a progressive contact device within said channel-shaped body,and' a resilient actuator for said progressive contact device afiixed to said bar within said channelshaped body.

. 7. The structure claimed in claim 6 wherein said cover is faced within with a lubricant-impregnated porous material serving as a bearing surface for. said bar. 8. The structure claimed in claim 6 wherein said cover is faced within with a lubricant-impregnated porous material serving as a bearing surface for said bar, and wherein said blocks are blocks of wood impregnated with lubricant.

- ,9. The structure claimed in claim 6 wherein said cover is faced within with a lubricant-impregnated porous material serving as a bearing surface for said bar, and wherein said blocks are blocks of wood impregnated with lubricant, and wherein said channel-shaped body is perforated to permit electrical connections to said progressive contact device, and is lined within with a flexible insulative sheet material coacting with the other recited elements to provide a substantially dustproof construction.

a 10. Thestructure claimed in claim 6 wherein said cover is faced within with a lubricant-impregnated porous material serving as a bearing surface for said bar, and wherein said blocks are blocks of wood impregnated with lubricant, and wherein said channel-shaped body is perforated to permit electrical connections to said progressive contact device, and is lined within with a flexible insulative sheet material coacting with the other recited elements to provide a substantially dustproof construction, and wherein a projecting end of said bar at one end of said body is adapted for connection to a rocking actuator, the other end of said body having a projecting spring leaf attached thereto, and adapted to be aflixed to an external support thereto,

whereby said device may bermounted to rockcto accommodate .the rocking motion of said actu1- ator.

11. The structure claimed in claim 10in which said progressive contact device comprises an insulative base affixed to said body and having a resistive coating thereon, a metallic strip supported above said resistive coating and depressible into contact therewith by said actuator, and a coating of elastic, deformable, resilient electrically conductive substance on the surface of said metallic strip facing said resistive coating. 112. The structure claimed in claim 6 in which said progressive contact device comprises an'insulative base affixed to said body and having a resistive coating thereon, a metallic strip'supported above said resistive coating and depressible into contact therewith by said, actuator and a coating of elastic, deformable, resilient, electrically conductive substance on the surface of said metallic strip facing said resistive coating.

13. The structure claimed in claim 6, including a rocking actuator, said bar being pivotally connected thereto at a point having arcuate motion, and a leaf spring projecting from said device and fixed to an external support, whereby to accommodate the rocking motion of said actuator.

- RAYMOND P. MORK.

REFERENCES CITED The following references are'of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,577,981 Otto Mar. 23, 1926 2,023,382 Schellenger Dec. 3, 1935 2,081,572 .Bagno May 25, 1937 2,215,124 Kock etal Sept. 1'7, 1940 2,254,366 Hammond Sept. 2, 1941 2,296,125 Traub Sept. 15, 1942 2,297,829 Hammond, Jr Oct. 6, 1942 2,333,477 Duston Nov. 2, 1943 FOREIGN PATENTS Number Country Date Great Britain Oct. 26, 1901 

